Landing gear by-pass valve



Jam 23, 1945. o. A. KEHLE LANDING GEAR BY-PASS'VALVE Filed April 9, 1943 5 Sheets-'sheet 1 Siwa/WIM Jan. 23, i945, Q A KEHLE v 2,367,682

LANDING GEAR BY-PASS VALVE Filed April 9, 1943 5 sheets-sheet 2 Uw I 27 36 l 36 44 4 44 j, M 45 42 m" d 4 f 'f FE1 :l g 1 l 46 45 44 an. 23, 1945. o. A. KEHLE LANDING GEAR BY-PASS VALVE Filed April 9, 1943 3 Sheets-Sheet 5 Lll i mented Jan. 23, 1945" LANDING GEAR BY-PASS VALVE Ottmar A. Kehle, Van Nuys, Calif., assignorto Adel Precision Products Corp., a corporation of California Application April 9, 1943, Serial No. t182.469 7 Claims. (Cl. 121-38) This invention relates to aircraft landing gear by-pass valves of the type shown in United States Letters Patent No. 2,267,284, issued December 23, 1941, to C. B. Livers, for controlling the llow o1' hydraulic uid in the landing gear actuating cyl'- inders and circuits so as to prevent evacuation oi' the upper end of the cylinders and cavitation of the pump and pressure lines during the lowy ering of the landing gear.

The primary object of the present invention is to provide a valve of the character described which embodies improvements over similar valves in that it is simplied as to the construction and arrangement of the ports, passages and various parts and elements thereof, consists of fewer working parts, is reduced as to weight, may be made at a lower cost, requires less servicing and parts replacement over longer periods of use, and is more efficient and reliable as to performance.

An important object of my invention is to provide a by-pass valve of the character described which is constructed, arranged and operated to prevent hydraulic locking of the piston valve thereof and consequent failure of the device to .cy-pass iiuid from one end or' the cylinder to the other, this objection being encountered with bypass valves heretofore used, wherein the piston valve must be given a considerable movement to open the by-pass port and wherein a cavity producing a vacuum is formed in the piston-valve Fig. '7 is a schematic view of a hydraulic system for actuating .landing gear showing my improved by-pass valve as it would appear when the landing gear is being extended and indicating how the iiuid is by-passed from the bottom to the top of the cylinder;

Fig. 8 is a schematic view similar to Fig. 7 but showing the by-pass valve as it would appear during retraction of the landing gear and the manner of the ow of pressure iiuid when the gear is retracted.

Referring to the drawings more specically, and particularly to the schematic illustrations in Figs. '7 and 8, it is seen that a by-pass valve A embodying my invention is connected in one of w the two cylinder lines B and C (for example line end of the cylinder to the upper end thereof.

bore before the piston is moved sufiiciently to open the icy-pass port.

With the foregoing objects in view, together with such other objects and advantages as may subsequently appear, the invention resides in the parts and in the combination, construction and arrangement of parts hereinafter described and. claimed, and illustrated by way of example in the accompanying drawings, inwhlch:

Fig. l is a sectional view of a by-pass valve embodying my invention;

Fig. 2 is a fragmentary sectional view showing the valve as it would appear when by-passing iiuid from one end of ,a hydraulic cylinder to the other;

Fig. 3 is a vertical sectional view taken on the line 3-3 of Fig. l;

Fig. 4 is a sectional view of a modiiied form of my invention;

Fig. 5 is a fragmentary sectional view of the modified valve shown in Fig. 4 as it would appear when by-passing Huid from one end of the cylinder to the other;

Fig. 6 is a vertical sectional view taken on the line 5 6 of Fig. 5;

When the 4-way valve D is operated to retract the landing gear, the pressure iiuid ilows through the by-pass valve to the lower end of the cylinder I as shown in Fig. 8.

It should be noted that inasmuch as the extension of the landing gear is largely effected by force of gravity, the piston of the cylinder expels the fluid from the lower end of the cylinder faster than the pump can supply pressure fluid to the upper end of the cylinder, but with my by-pass valve arranged in the circuit as shown in Figs. 7 and 8 the iiuid from the lower end of the cylinder is by-passed to the upper end of the cylinder, thereby preventing evacuation of the cylinder and any objectionable operation, or operation failure which would obviously attend such a disproportionate ow of fluid.

In addition to by-passing fluid as aforesaid, the valve A lacts to ydirect fluid from the lower end of the cylinder through the 4-way valve D into the return line H when the pressure oi' the operating uid above the piston in the cylinder E exceeds the pressure of the uid discharging from the lower end of the cylinder as is the case when the gravitational movement of the landing gear ceases and the pressure uid operates the cylinder to move the landing gear into its vconipletely 4extended position where it becomes mechanically locked in a manner well known to those skilled in the art, but not here shown.l

Reference is now had to Figs. l. 2 and 3, wherein the valve A is shown as including an elongated body member I having a bore 2 extending longitudinally therethrough. At one end of this bore is a connection fitting 3 screwed into a threaded enlargement 4 for connection of a section of the cylinder line B therewith, while at the other end of the bore a similar fitting 5 is screwed into a threaded enlargement 5 to provide for the connection thereto of the other section of the cylinder line B which leads tothe lower end of the cylinder as shown in Fig. '1.

A piston valve 1 having' a working fit in the bore 2 is provided at one end with an integral conical .valve member 8 adapted to seat against an annular seat 9 of smaller diameter than the piston proper and formed on the inner end of the fitting 5.

When pressure fluid is directed through the fitting 3 into the bore 2 for retractng the landing gear, the piston valve 1 is shifted by the pressure fluid so that the valve end 8 seats on the seat 9, moving from position shown in Figs. 2 and 'l into position shown in Figs 1 and 8.

When thus shifted the piston uncovers a passage I which opens into one end of the bore 2 and leads through the body to the other end of said bore, so that pressure fluid by-passes the then seated piston valve and will flow out through the fitting to the lower end of the cylinder E. In this connection it is seen that the fitting 5 has radial ports II affording communication between the passage I0 and the bore through the fitting 5, or generally speaking, a connection between the bore 2 and passage I0.

As the piston valve 1 proper, apart from the valve member 8 thereon, has a greater diameter than the seat 9, it is apparent that the pressure fluid will hold the piston valve seated while the passage I0 is open for delivery of pressure fluid to the lower end of the cylinder. A check valve I2 of the disk type is provided in the passage I0 adjacent the end thereof opening into the boreI near the fitting 3, said valve seating against a seat I3 in a direction toward the bore 2 so that when the passage I0 is uncovered by the piston valve 1, as shown in Figs. 1 and 8, the pressure fluid will open the valve against the action of the valve spring I4. This Spring is engaged at its ends with an adjustable screw plug I4 and the valve I2 and is tensioned so that the valve will be unseated byl the normal pumpu pressure for actuating the cylinder E.

When the landing gear is unlatched, in a manner not here shown, preparatory to its extension, the 4-way valve being set as shown in Fig. '1, so that pressure fluid will flow through the line C to the top of cylinder E, the landing gear will gravitate as indicated in Fig. '1 toward its extended position and force the uid from the lower end of the cylinder E at a greater rate than that of the flow of operating fluid entering the top of the cylinder from the pump. When the landing gear begins its descent, the piston 1 is closed as shown in Fig. 8, but as soon as the return fluid commences to flow from the lower end of the cylinder through line B and fitting 5, the piston 1 will be shifted so as to unseat the valve member 8 thereon and will assume the position shown in Fig. '1, whereby fluid will flow through the bypass line J, thence through line C to the top of the cylinder so that the proper volume of fluid ascacaa will be maintained above the piston during the extension of the landing gear.

When the piston 1 is moved'to unseat the valve 8 as shown in Fig. f7, it opens a port I5 leading from the bore 2 adjacent the'valve seat 9, into a nipple I6 formed on the body member I. Screwed into this nipple is a tubular fitting I1 to which is coupled a connection fitting I8 connecting the by-pass line J to the cylinder line C.

YIn the fitting I1 is a valve seat I8 for a spring loaded by-pass check valve of the disk type which seats toward the nipple. Inasmuch as the pressure of the return fluid from the cylinder during the gravitational extension of the landing gear is greater than that of the pressure fluid (plus the force of the spring 2 I) operative to seat the valve 20 due to the greater rate of egress than ingress of fluid relative to said cylinder, the valve 20 will be opened to allow the by-passing of the fluid from the lower to the upper end of the cylinder as hereinbefore pointed out.

As soon as the gravitational movement of the landing gear ceases, as takes place before the movement of the gear into fully extended position in which it becomes automatically latched and held, the pressure of the return fluid becomes less than that of the pressure fluid above the valve 20 and in the upper end of the cylinder E, whereupon the valve 2U is pressure seated and the piston in the cylinder E is moved to thrust the landing gear into finally extended position and latch it in such position. During this operation the fluid returned from the lower end of the cylinder E into'the bore 2 builds up a pressure in said bore until a relief check valve 22 of the ball type controlling a passage 23 through the piston 1, is forced open and the return fluid then passes through the piston passage 23, bore 2, fitting 3, and back to the reservoir (not shown). A spring 4G 2l having a tension adjusting means 25 is set to hold the valve 22 seated while the pressure of the fluid returning during gravitational movement of the landing gear, is greater than the pump operating pressure against the upper side of the piston and the valve 20. For this reason the valve 28 will be unseated during gravitational movement of the landing gear into extended position while the relief valve 22 remains seated. However, when the pressure of the operating fluid in line C and the upper end of the cylinder exceeds thatvof the return fluid, following gravitational movement of the landing gear, this higher pressure plus the force of the spring 2| holds the valve 20 seated while the return fluid pressure builds up to a pressure sufficient to open the relief valve 22 in the piston.

t should be noted that the conical valve memoer yil on the piston` 1, and the valve seat il are arranged so that the annular space formed in the bore 2 around the valve member 8 when the latter is being seated, opens into the port I5 leading to the valve 20 thereby preventing the trapping of fluid in the bore between the piston 1 and valve seat 9.

It should be noted that passage I0 is formed by making a bore Illa longitudinal from one end of the body member I to parallel the bore 2 and which terminates adjacent but spaced from the opposite end of the body member, then making bores I0b and IIlc from one side of the body member so as to intersect the bore Illa and extend at right angles thereto, to points of communication with the bore 2 near the ends of the latter.

The outer ends of bore IIIa and IIIc are closed 'by plugs |0d and |06 respectively, whereas the outer end of bore |b is closed by the adjustable plug I for regulating the valve spring I4 for Referring now to Figs. 4, 5 and 6, it is seen thata modified form of my invention includes a valve body member 21 of substantially triangular outline instead lof the generally rectangular body member shown in Figs. 1, 2 and 3. This form of my improved valve has substantially theYY same number of working parts and operates in the same manner as the valve assembly shown in Figs. l, 2. 3, 7 and 8, but differs as to the construction and location of certain parts to better adapt the unit for heavy duty. 'I'he body member 2l as here shown is provide with a large bore 28 extending longitudinally therethrough and provided with connection ttings 29 and 3|! at its ends. also a piston 3| reciprocable in said bore. The piston has an integral conical valve member 32 at one end adapted to seat against a seat 33 on the inner end of the tting 3U.

Extending diagonally from a point near the inner end of the tting 29 where it opens into the bore d is a passage 3d which opens in a valve chamber formed as bore leading from one end of the body member, and extending parallel tothe bore A port 3G affords communication between the chamber and the portion of bore 28 which communicates with the interior of the iii.- ting through radial openings 3l.

A cylindrical valve cage 3B is threaded into the chamber and has an annular valve seat 39 provided interiorly thereof for a poppet check valve The valve iii has an integral cylindrical guide portion di arranged in a working nt within the cage so as to be unseated by pressure fluid entering chamber from the passage 3d. A spring is interposed between the valveid and a plug d3 turned in the outer end of the cage 38.

The guide :il of the valve has radial ports d@ affording communication oi the interior ofthe guide with the interior of the cage 35 which latter is likewise provided with ports i5 (see Fig. 6) opening into a counterbore it in the 'bore 28 to establish communication between the interior oi the cage and the port 35.

When pressure fluid is directed through the fitting 29 into the bore 2 for the purpose of retracting Ithe landing gear in the same manner as shown in Fig. 3, said iiuid flows through the passage 34 into chamber 35 and opens the valve 40, then passes through the cage 38, ports d5, counterbore dt, port 36, radial openings 37, fitting 30, and to the hydraulic cylinder. The piston 3| is moved by the pressure iiuid in the bore 28 so as to seat the valve 32 and hold it seated as shown in Fig. 4 during the dow of pressure uid to retract the landing gear. y

When return iluid from the lower end of the hydraulic cylinder is delivered through fitting 30 during gravitational extension of the landing gear. the pressure of said iluid shifts the piston 3| so as to unseat the valve 32. This movement of the piston exposes a by-pass check valve 48 mounted in a fitting 49 on the body 21 as shown in Fig. 5. This fitting is adapted to be connected to a cylinder line leading to the top of the cylinder in the same manner as shown in Figs. '1 and 8. As the ratelof movement of the piston is greater than the rate of iiow of pressure fluid in the line to which the fitting 49 is connected during gravitational movement of the landing gear, it is seen that the pressure in this fitting is reduced below that of the return iiuid beneath the -by-pass -valve -48 wherefore said valve will open and allow the fluid to enter the top of the cylinder as shown in Fig. 7.

A relief check valve 5|) is mounted in a bore 5| through the piston 3| to engage a seat y52 in said bore and operates in the same manner as the relief valve shown in Figs. 1 and 2. The valve 50 is of the disk poppet type'as are the valves 40 and 43 and the springs 53 and 54 for these valves are relatively tensioned in the same manner as the corresponding springs in the first described form of my invention. Ports in theyalve 50 and similar ports 56 in the valve 48 provide for free flow of uid when said valves are unseated.

As this modified lform of my invention operates in the saine manner as the first described form further description as to operation is deemed unnecessary. However, it should be noted that the diagonally extending passage 34 and associated ports and passages, with the valve 40`located at the opposite end of the valve body member 21 compared to the valve shown in Figs. 1 and 2, and the body 21 of substantially triangular fonm, I am able to provide relatively large ports and passages and a. heavy duty by-pass valve assembly as a small, compact unit of comparatively light weight.

It is important to note that in bothforms of my icy-pass valve as here shown the piston valve upon being cracked or slightly unseated, will immediately esta'blish communication of the by-pass port with the portion of the main bore into which the return iiuid flows from the lower end of the hydraulic cylinder upon extension of the landing gear. 'Therefore the lay-passing from the lower end of the cylinder to the upper 'end thereof willv ate icy-passing action is assured by employing a piston valve construction in which an annular space is defined around that portion of the piston valve which lies between the by-pass port and the seat lfor the piston valve when the latter is seated with this space aording an immediate communication between said port and said seat upon the initial minute movement of the piston valve. In other words, the piston valve does not at any time close the by-pass port and it is only necessary to give but a slight movement of this valve to expose the fby-pass check valve to the pressure of -bypassed uid and thereby start the flow off such fluid to the top of the hydraulic cylinder.

While I have shown and described specic embodiments of my invention I do not; limit myself to the exact details of vconstruction set forth, and the invention embraces such changes, modifications and equivalents of the parts and their rformationand arrangement as come within the purview of the appended claims.

I claim:

1. In a hydraulic by-pass valve, a valve body having a uid passage extended therethrough, connection means on said body for coupling said passage in series with a fluid line for conducting pressure nuid to and returning uid from one end of a hydraulic cylinder, a by-pass connection on said body for coupling said passage with a second duid line for conducting pressure duid to and returning duid from the other end of said cylinder, a by-pass check valve means arranged to seat and close said by-pass connection responsive to pressure fluid in said second duid line and to unseat and open said by-pass connection when the duid returned from said one end of the cylinder to said passage reaches a pressure greater than the pressure of the fluid seating said check valve means, said body having a bore therein communicating at its ends with said passage, a piston valve arranged in said bore to move into position to shut oil.' communication between said by-pass connection and said passage responsive to the passage of pressure fluid through said passage for delivery to said one end of the cylinder, and to move into position affording communication of said by-pass connection with said passage responsive to the return of duid from said one end oi' the cylinder to said passage, a check valve means arranged to close said passage at a point between the points of communication oi said bore with the passage responsive to the flow of return duid into said passage from said one end of the cylinder, said piston valve having a relief port extended therethrough for conducting return duid from said passage through said bore and out of said valve body while the check valve in said passage is closed; and a relief check valve arranged in said port to be seated and close said port while the pressure of return duid in the passage is greater than the pressure of the operating duid holding the by-pass check valve seated, and to be opened by pressure of the return duid when the pressure of the return fluid is less than that of the operating duid seating said by-pass check valve, there being in said bore a valve seat of smaller diameter than said piston valve, the latter having a frusto-conical end engageable with said valve seat.

2. In a hydraulic by-pass valve, a valve body of substantially triangular outline having a bore extended therethrough, connection means on said body for coupling the bore in series with a duid line for conducting pressure duid to and returning duid from one end of a hydraulic cylinder, a passage in said body having its ends arranged to open into the end portions of said bore, a check valve intermediate the ends of said passage arranged to open incident to the dow of pressure duid into said passage from said bore and to close responsive to the dow of return duid into said passage from said bore, a by-pass connection on said body having communication with said bore, and adapted for coupling said bore with another duid line through which pressure duid is directed to and duid is returned from the other end of the hydraulic cylinder, whereby return fluid may be directed from said one end of the cylinder to the other end of the cylinder; a by-pass check valve arranged to be seated to close said by-pass connection, by pressure of duid in the duid line coupled to said by-pass connection and to be unseated and open said by-pass connection when the pressure of return duid in said bore exceeds the pressure of the duid in the duid line coupled to said by-pass connection; a valve seat in said bore at a point between the ends thereof 3 a piston valve in said bore operat ing responsive to the introduction of `pressure duid into said bore for delivery to said one end of the cylinder through said passage, to engage said valve seat and shut off communication of the bore with said by-pass connection and by-pass check valve, and to be unseated and open said bore to communication with said by-pass connection when return fluid is delivered to said bore from said cylinder; a port in said piston valve aiIording the passage of return duid through said bore and away from said body, while the check valve in said passage is closed; and a check valve controlling said port so that the port will be open only when the pressure of the duid seating the by-pass check valve is greater than the pressure of the return duid, said passage extending obliquely relative to said bore and having in it a valve chamber in which the drst named check valve is mounted.

3. In a hydraulic by-pass valve, a valve body having a bore extended therethrough, connection means on said body for coupling the bore in series with a duid line for conducting pressure duid to and returning duid from one end of a hydraulic cylinder, a passage in said body having its ends arranged to open into the end portions of said bore, a check valve intermediate the ends of said passage arranged to open incident to the dow of pressure fluid into said passage from said bore and to close responsive to the dow of return fluid into said passage from said bore, a by-pass connection on said body having communication with said bore, and adapted for coupling said bore with another duid line through which pressure duid is directed to and duid is returned from the other end of the hydraulic cylinder, whereby return fluid may be directed from said one end of the cylinder to the other end of the cylinder; a

by-pass check valve arranged to be seated to close said by-pass connection by pressure of duid in the duid line coupled to said by-pass connection and to be unseated and open said by-pass connection when the pressure of return fluid in said bore exceeds the pressure of the fluid in the duid line coupled to said by-pass connection; a valve seat in said bore at a point between the ends thereof; a piston valve in said bore operating responsive to the introduction of pressure fluid into said bore for delivery to said one end of the cylinder through said passage, to engage said valve seat and shut oi communication of the bore with said by-pass connection and bypass check valve, and to be unseated and open said bore to communication with said by-pass connection when return duid is delivered to said bore from said cylinder; a port in said piston valve affording the passage of return duid through said bore and away from said body, while the check valve in said passage is closed; and a check valve controlling said port so that the port will be open only when the pressure of the duid seating the by-pass check valve is greater than the pressure of the return duid, said valve seat being of a smaller diameter than said bore, and the piston valve having a frusto-conical end to seat on said valve seat, said port opening through the center of said frusto-conical end.

4. In a hydraulic by-pass valve for controlling the dow of duid in a hydraulic cylinder for aircraft landing gear which responds to gravitational forces while being extended, a valve body having a bore therein, means affording the connection of one end of said bore with a duid line for conducting duid to and from said bore, means for connecting the other end of said bore with a cylinder line for conducting duid from said bore to the lower end of the cylinder for retracting the landing gear, through which cylinder line return duid dows to said bore upon the extension of the landing gear, a by-pass port opening into said bore, a by-pass connection aording the connection of said port with a second cylinder line for conducting iiuid to and returning it from the upper end of said cylinder, a by-pass check valve in said connection arranged to close said port responsive to pressure of iiuld derived in said connection from the second cylinder line and to open said port responsive to the pressure of return iiuid in said bore and port as derived upon gravitational extension of said landing gear, whereby fluid returned from the lower end of the cylinder is by-passed through said bore, said port and said connection to the upper end of the cylinder, a pressure uid passage in said body having its ends opening into said bore adjacent the ends of the latter for conducting pressure iiuid through the body for delivery to the lower end of the hydraulic cylinder to retract the landing gear, a check valve in said passage opening responsive to pressure fluid delivered to said bore through first named end of said bore and closing responsive to return fluid from the cylinder connected end of the bore, a valve seat in said bore between the by-pass port and the point where the pressure passage discharges pressure iluid into saidv bore, a piston valve mounted in said bore to engage said seat and shut oir communication between said by-pass port and the cylinder connected end of said bore responsive to pressure uid delivered to said bore through said one end of the bore and operating to open said cylinder connected end of the bore to communication with said seat responsive to the pressure of fluid returned to the bore during gravitational extension of the landing gear; saidpiston, said seat and said by-pass port being constructed and arranged so that a portion of the bore surrounding the piston aiords communication of the port up to the point where the piston engages its seat whereby upon initial unseating movement of said piston the cylinder connected end of the bore is immediately brought into directy communication with said port, a relief passage within the'body aording the discharge of return iiuid from the cylinder connected end of the bore through the other end of the bore; and a normally closed check valve in said relief passage set to/open only when the seating pressure on the by-pass check valve exceeds that of the return uid pressure in the cylinder connected end of the bore.

5. In a hydraulic by-pass valve for controlling the iiow of fluid in a hydraulic cylinder for aircraft landing gear which responds to gravitational forces while being extended, a valve Abody having a bore therein, means aording the connection of one end of said bore with a iiuid line for conducting fluid to and from said bore, means for connecting the other end of said bore with a cylinder line for conducting yiluid from said bore to the lower end of the cylinder for retracting the landing gear, through which cylinder line return fluid ows to said bore upon the extension of the landing gear, a by-pass port opening into said bore, a by-pass connection alfording the connection of said port with a second cylinder line for conducting fluid to and returning it from the upper end of said cylinder, a by-pass check valve in said connection arranged to close said port responsive to pressure fluid derived in said connection from the second cylinder line and to open said port responsive to the pressure of return fluid in said bore and port as derived upon gravitational extension of said landing gear, whereby uid returned from the lower end of the cylinder isby-passed through said bore, said port and said connection to the upper end of the cylinder; a pressure iiuid passage in said body having its ends opening into said bore adjacent the ends of the latter for conducting pressure iluid through the body for delivery to the lower end of the hydraulic cylinder to retract the landing gear, a check valve in said passage opening responsive to pressure iluid delivered to said bore through first named end of said bore and closing responsive to return fluid from the cylinder connected end of the bore, avalve seat in said bore between the by-pass port and the point where the pressure passage discharges pressure uid into said bore, a piston valve mounted in said bore to engage said seat and shut oi communication between said by-pass port and the cylinder connected end of said bore responsive to pressure iuid delivered to said bore through said one end of the bore and operating to open said cylinder connected end of the bore to communication with said seat responsive to the pressure fluid returned to the bore during gravitational extension oi the landingcgear; said piston, said seat and said bypass port being so constructed and arranged that a portion of the bore surrounding the piston affords communication of the port up to the point where the piston engages its seat whereby upon initial unseating movement of said piston the cylinder connected end of the bore is immediately brought into direct communication with said port, a relief passage within the body affording the discharge of return fluid from the cylinder connected end of the bore through the other end' of the bore; and a normally closed check valve in said relief passage set to open `only when the seating pressure on the by-pass check valve exceeds that of the return iiuid pressure in the cylinder. connected end of the bore, said relief passage extending axially through the piston and being exposed through the valve seat to the cylinder connected end of the bore when the piston valve is seated.

6. The invention as set forth in claim 2 wherein the piston valve has a conical end portion to engage the valve seat therefor and the portion of the bore between the seated portion of said conical end portion and said by-pass connection is open whereby upon initial movement of the piston away from its seat the return :duid pressure has immediate access to said by-pass check valve.

7. In a hydraulic by-pass valve for controlling the flow of uid in a hydraulic cylinder for aircraft landing gear which responds to gravitational forces while being extended, a valve body having a bore therein, means aiording the connection of one end of said bore with a fluid line for conducting fluid to and from said bore, means for connecting the other end of said bore with a cylinder line for conducting iluid from said bore to the lower end of the cylinder for retracting the landing gear, through which cylinder line return fluid flows to said bore during extension of the landing gear, a by-pass port opening into said bore, a second cylinder line having a by-pass connection affording the connection of said port with said bore for conducting fluid to and returning it from the upper end of said cylinder, a by-pass check valve in said connection arranged to close said port responsive to pressure of iiuid derived in said connection from the second cylinder line and to open said port responsive to the pressure of return iluid in said bore and port as derived upon gravitational extension of said landing gear, whereby iluid returned from the lower end of the cylinder is by-pa'ssed through said bore, said port and said connection to the upper end of the cylinder, a pressure fluid passage in said body having its ends opening into said bore adjacent the ends of the latter for conducting pressure fluid through the body for delivery to the lower end of the hydraulic cylinder to retract the landing gear, a check valve in said passage opening responsive to pressure fluid delivered to said bore through rst named end of said bore and closing responsive to return fluid from the cylinder connected end of the bore, a valve seat in said bore between the by-pass port and the point where the pressure passage discharges pressure uid into that end portion oi.' said bore, which is connected with the lower end of the aforesaid hydraulic cylinder, a piston valve4 mounted in said bore to engage said seat and shut of! communication between said by-pass port and the cylinder connected end of said bore responsive to pressure duid delivered to said bore through said one end of the bore and operating to open said cylinder connected end of the bore to communication with said seat responsive to the pressure oi iluid returned to the bore during gravitational extension of the landing gear; said piston, said seat and said by-pass port being constructed and arranged so that upon initial unseating movement oi said piston the cylinder connected end of the bore is immediately brought into direct communication with said port, a relief passage within the body affording the discharge oi return uid from the cylinder connected end of the bore through the other end of the bore; and a normally closed check valve in said relief passage set to open only when the seating pressure on the by-pass check valve exceeds that of the return uid pressure in the cylinder connected end of the bore,

OT'I'MAR A. KEHLE. 

